The excitation of obliquely propagating fast Alfvén waves at fusion ion cyclotron harmonics

The theory of the magnetoacoustic cyclotron instability, which has been proposed as a mechanism for suprathermal ion cyclotron harmonic emission observed in large tokamaks, is generalized to include finite parallel wave number k ∥. This extension introduces significant new physics: the obliquely propagating fast Alfvén wave can undergo cyclotron resonant interactions with thermal and fusion ions, which affects the instability driving and damping mechanisms. The velocity–space distribution of the fusion ions is modeled by a drifting ring, which approximates the distribution calculated for the emitting region in tritium experiments on the Joint European Torus (JET) [Cottrell et al., Nucl. Fusion 33, 1365 (1993)]. Linear instability can occur simultaneously at the fusion ion cyclotron frequency and all its harmonics when the fusion ion concentration is extremely low, because the finite k ∥ gives rise to a Doppler shift, which decouples cyclotron damping due to thermal ions from wave growth associated with fusion ions. Doppler shifts associated with finite k ∥ may also be related to the observed splitting of harmonic emission lines.